1. Technical Field of the Invention
This invention relates to a silicon photovoltaic device. More particularly, this invention relates to a silicon photovoltaic device made using particulate silicon which may be polycrystalline.
2. Background Art
The production of photovoltaic devices or solar cells from silicon historically has involved the use of high purity, single crystal silicon which is doped to produce either a p-type or n-type material. The doped crystal is then sliced into discs or wafers and, depending upon the previous doping, either an electron donor or an electron acceptor is diffused into selected regions to form p-n junctions. Electrodes are then applied respectively to the diffused and nondiffused regions.
This technique, while resulting in the production of an acceptable photovoltaic device, is expensive and time-consuming, particularly in the cost of diamond sawing the silicon to produce wafers. Furthermore, the size of the cell is limited by the size of the wafer which is, in turn, limited by the size of the single crystal ingot. While the size may be increased using cast silicon sheet, the costs are commensurately increased by the use of such techniques.
Much attention has, therefore, been directed to the possible use of particulate and/or polycrystalline materials. The use of such materials was earlier investigated in the production of cadmium sulfide solar cells. Tanos U.S. Pat. No. 3,480,473 teaches a method of making an improved photovoltaic cell from polycrystalline cadmium sulfide by etching the surface of a cadmium sulfide film vacuum deposited on a molybdenum substrate. The vacuum deposited cadmium sulfide forms an n-type layer on the substrate. A barrier layer of copper is then applied forming a p-type layer.
Yamashita U.S. Pat. No. 3,615,877 also describes a method of making a photovoltaic cell employing powdered polycrystalline cadmium sulfide. The method involves coating a substrate having an electrode thereon with a composition containing cadmium sulfide and then firing the coated substrate to obtain a sintered film. A metal, such as copper, is electroplated to at least a portion of the surface to convert the surface to p-type thereby forming a p-n junction.
Particulate silicon has also been used to form solar cells. Paradise U.S. Pat. No. 2,904,613 describes the use of 2 mm silicon particles having an electron acceptor material previously diffused a predetermined distance into the entire outer surface of the particle. The particles are coated with an insulator and then applied to a reinforcing plate. The exposed side of the particle layer is then etched until the faces of the crystallites are exposed and the diffused layer removed, thereby exposing "N" silicon. The exposed silicon is then plated with nickel, and solder is then used to sweat a continuous base plate to the negative surface. The reinforcing plate is then removed, thereby exposing the "P" layer to which an electrode is applied using selective etching techniques.
Kilby et al U.S. Pat. No. 4,021,323 also discloses the use of particulate silicon to form a type of solar cell using n-type and p-type spheres which have surface diffusions respectively forming p-n junctions in each sphere. The spheres are coated with a metal layer and then an insulating layer and then pressed onto a wax covered substrate. The spheres and the substrate are then flooded with a layer of insulating compound. The upper surface of the spheres is then etched to expose the cores of the spheres. A thin layer of aluminum is then deposited, followed by a layer of material for strength and protection. The wax covered substrate is then removed from the opposite side of the device, and the insulating layer on the surface of the spheres is removed to expose the top portion of the metal covered spheres. The device is intended for use with an electrolyte wherein generation of photovoltaic current will result in the production of hydrogen which is then stored for future use.
While the foregoing uses of particulate and polycrystalline material undoubtedly represents improvement in the cost of manufacture of solar devices over single crystal devices, there still remains a need for elimination of costly preprocessing, e.g., doping by diffusion, of individual silicon particles prior to their incorporation into an array or device.